1. Field of the Invention
This invention relates to radio frequency (RF) chokes used with radio frequency and microwave frequency signals in general and more particularly to a miniature wideband RF choke that has a small package size and that can be manufactured at low cost.
2. Description of the Related Art
RF chokes are used in various devices. The RF choke separates an RF signal from a DC signal or a single-phase AC signal by presenting a low impedance to the DC or low frequency (60 Hz) AC signal. At the same time, the RF choke presents a high impedance to a RF signal, which typically has a frequency range of 5 to 1000 MHz. The DC or low frequency AC signal is shunted through the RF choke, while the RF signal is blocked. A perfect RF choke would pass all of the DC or low frequency AC signal through the RF choke while blocking all of the RF signal.
Referring to FIG. 1, a schematic diagram of an RF choke 20 is shown. RF choke 20 has an input port IN and an output port OUT. Inductor L has one end connected to input port IN and another end connected to output port OUT. The inductor can be a wire wound on a ferrite core. The parasitic capacitance of the inductor is shown as capacitor C. The loss of the ferrite core and the resistance of the wire are shown as resistor R. For good performance at low frequencies, the inductance should be large. Unfortunately, when the inductance is large, the parasitic capacitance is also large and the parasitic resistance low. The result is that the electrical performance of the RF choke is poor at high frequencies.
In order to increase the bandwidth performance of RF choke 20 over a larger frequency range, a second inductor in series can be added. Referring to FIG. 2, a schematic diagram of a wideband RF choke 30 is shown. Wideband RF choke 30 has an input port IN and an output port OUT. Inductor L1 has one end connected to input port IN and another end connected to node 32. Inductor L2 has one end connected to output port OUT and another end connected to node 32. The inductor L1 and L2 can be wires wound on ferrite cores. The parasitic capacitance of the inductors are shown as capacitors C1 and C2. The loss of the ferrite cores and the resistance of the wires are shown as resistors R1 and R2. Inductor L1 is selected to be large enough for proper low frequency operation. Inductor L2 is selected to be small for high frequency operation. Since inductor L2 has a small value, the parasitic capacitance C2 is small and the parasitic resistance R2 is high. Therefore RF choke 30 has good performance at both high and low frequencies.
Referring to FIG. 3, a prior art RF choke assembly or package 40 is shown. RF choke assembly 40 has a plastic housing 42 with a top surface 43 and a cavity 44. Six metal leads 46 are attached to top surface 43. A ferrite binocular core 48 and a ferrite single core 50 are mounted in cavity 44. Wire 52 has ends 52A and 52B. Wire 52 is wound on cores 48 and 50 and the ends attached to respective leads 46. Core 48 forms inductor L1 and core 50 forms inductor L2. RF choke assembly 40 has typical dimensions of 0.310 inches in length by 0.280 inches in width by 0.112 inches in height. RF choke 40 has an area of 0.0868 square inches. RF choke assembly or package 40 is typically soldered onto another printed circuit board.
Unfortunately, RF choke assembly or package 40 takes up excessive space when it is mounted on a printed circuit board. The mounting of the cores side by side results in a large package. The mounting of the cores and winding of the wire are manual operations that are difficult to automate. It is desirable, in order to reduce cost, to automate as much of the assembly process as possible.
While RF chokes have been used, they have suffered from being too large, expensive, difficult to assemble and not easily manufactured using automated equipment. A current unmet need exists for a wideband RF choke that has a smaller size, can be assembled at a low cost and that can be manufactured using automated equipment.